When introducing any desired fatty acids to the 1- and 3-positions using a lipase that is selective for the 1- and 3-positions, it is general to employ a method of conducting acidolysis with an objective fatty acid, or a method of conducting ester interchange of an objective fatty acid ester. With those methods, however, a much excessive amount of fatty acid or ester must be used. Also, from the viewpoint of probability theory, a part of original fatty acids in the 1- and 3-positions remain. It is therefore relatively difficult to obtain a lipid having the objective structure at a high purity.
Further, because lipases have the activity greatly changeable depending on the chain lengths of fatty acids and the activity upon long-chain fatty acids, such as DHA, are extremely weak, a lipid having the objective structure cannot be obtained in some cases.
Lipases cause reactions such as hydrolysis, interesterification, acidolysis, and esterification. However, there are lipases having various characteristics, and all kinds of lipases do not develop the same degree of reactivity under the same conditions in all of the reactions. For example, the use of a 1,3-lipase having a strong reactivity for acidolysis is advantageous in substitution of fatty acids in the 1- and 3-positions, but when that lipase does not have a sufficient reactivity for DHA, etc., a much difficulty arises in substitution of a TG having DHA in the 1- and 3-positions for another objective fatty acid. Of course, the objective substitution can be achieved by using a lipase having a high activity not only for acidolysis, but also for DHA. Such a lipase is, however, difficult to obtain or is very expensive. Hence industrial use of that lipase is disadvantageous.